(a) Field of the Invention
The present invention relates to a copper alloy suitable for use as a material of a heat exchanger such as a radiator which cools cooling water circulated through an automotive engine, automotive air heater, and various other industrial and household heat exchangers. More particularly, the present invention relates to a copper alloy suitable for use as the material of constituents of a heat exchanger such as the tube plates, tanks and tubes.
(b) Description of the Prior Art
A heat exchanger is composed of tanks, tube plates, tubes and fins. Fins are usually made of a heat-resistant copper having a high purity approximating that of pure copper, whereas the tanks, tube plates and tubes are made of a material such as Cartridge Brass 70% (C2600) or Yellow Brass 65% (C2800), in order to cope with demands for workability, strength and economy.
In general, a copper alloy to be used as the material of the heat exchanger is required to meet the following requirements.
As a heat exchanger in which a coolant, wherein generally polyethers are added to water, is circulated, the constituents of the heat exchanger must have a high corrosion resistance so as to prevent internal corrosion by such a coolant.
It is also preferred that the constituents of a heat exchanger have sufficiently high resistance to resist any external corrosive condition such as a salt-containing atmosphere.
High workability is required for the material of the tanks and the tube plates because they have to be made by deep drawing. In general, the material of such parts is evaluated in terms of the Erichsen value in the Erichsen deep drawing cup test. The material of the tubes also is required to have a high workability approximating that of brass because the tubes are often formed through a complicated rock seam tube process.
The materials of these structural parts also are required to have high mechanical strength which well compares with that of brass. High strength is required particularly when the tank and the radiator core are fixed to each other mechanically, because the required reliability of the mechanical connection may not be obtained when the strength of the material is low. Furthermore, tubes are required to have high rigidity because inferior rigidity will make the work for forming the tubes difficult.
Superior solder wettability is also an important requisite because a heat exchanger, in particular a radiator, employs many portions connected by soldering.
In general, brass exhibits a rather inferior resistance to stress corrosion cracking which often results in a leakage of an internal fluid. Usually, therefore, an annealing is effected to remove any residual stress, thereby preventing occurrence of stress corrosion cracking. Unfortunately, however, annealing alone cannot completely eliminate stress corrosion cracking.
On the other hand, there is a current trend for use of reinforced plastics as materials of constituents of a heat exchanger, particularly as the material of the tank. When such a resinous tank is used, the peripheral Portion of the tube plate is bent to form a concave groove opening to the outside, an elastic seal member is provided in said concave groove to insert the flange portion of the resinous tank, and the upper portion of the outer side of the concave groove is bent inwards to securely attach the tank. In such a case, it is not permissible to effect annealing after the assembly because of the use of the plastic as the tank shell material. In consequence, a considerably high level of residual stress remains in the tube plate, presenting a high sensitivity to stress corrosion cracking. In order to obviate this problem, it is a common practice to use a separately prepared member made of a stainless steel, which is bent and securely attach the tank to the tube plate. This solution, however, is disadvantageous from the view point of economy.
Under these circumstances, discussion is made in the specification of U.S. Pat. No. 4,741,394 as to the use of an alloy containing 15 to 38 Wt % of Zn and 0.05 to 1.5 wt % of Si. Unfortunately, however, there still is a practical limit in the effect of prevention of stress corrosion cracking even with that material. In addition, the strength is undesirably reduced when the Zn content is decreased to improve resistance to stress corrosion cracking, while an enhancement in the strength by addition of other elements tends to result in a reduction in workability.
The same problems are encountered also with the materials of the tubes and the copper alloy tank material. In case of tube materials, dezincification corrosion of the inner tube surface by a coolant can be avoided by addition of Sn and P to the tube material but dezincification which tends to be caused on the external side of the tube by road salt cannot be avoided unless the Zn content is reduced. Such a reduction in the Zn content may impair the strength significantly. Addition of additives or elements for the purpose of compensating for the reduction in the strength tends to cause a reduction in the workability.